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Collection and Storage Using Salinity-Gradient Technology

A salinity gradient solar pond is an integral collection and storage device of solar energy. By virtue of having built-in thermal energy storage, it can be used irrespective of time and season. In an ordinary pond or lake, when the sun's rays heat up the water this heated water, being lighter, rises to the surface and loses its heat to the atmosphere. The net result is that the pond water remains at nearly atmospheric temperature. The solar pond technology inhibits this phenomena by dissolving salt into the bottom layer of this pond, making it too heavy to rise to the surface, even when hot. The salt concentration increases with depth, thereby forming a salinity gradient. The sunlight which reaches the bottom of the pond remains entrapped there. The useful thermal energy is then withdrawn from the solar pond in the form of hot brine. The pre-requisites for establishing solar ponds are: a large tract of land (it could be barren), a lot of sun shine, and cheaply available salt (such as Sodium Chloride) or bittern. http://teriin.org/division/eetdiv/docs/ca02_1.htm

How does a solar pond work? Most people know that fluids such as water and air rise when heated. The salinity gradient stops this process when large quantities of salt are dissolved in the hot bottom layer of the body of water, making it too dense to rise to the surface and cool. Generally, there are three main layers. The top layer is cold and has relatively little salt content. The bottom layer is hot -- up to 100°C (212°F) -- and is very salty. Separating these two layers is the important gradient zone. Here salt content increases with depth. Water in the gradient cannot rise because the water above it has less salt content and is therefore lighter. The water below it has a higher salt content and is heavier. Thus, the stable gradient zone suppresses convection and acts as a transparent insulator, permitting sunlight to be trapped in the hot bottom layer from which useful heat may be withdrawn or stored for later use.